A film that includes at least one coating that contains a modified thermoplastic olefin homo-polymer or copolymer has been known for a long time. Products made of polyethylene (PE), for example, and especially those made of low density polyethylene (LDPE), are trash bags, shrink films and agricultural films. In addition, numerous multilayered films are known that have three, five or seven layers, for example, from which one or several contain PE or PP (polypropylene).
It can be generally said that the application area of films containing olefin homo or copolymers is very large. One of the fields of use is also the pipe lining process for trenchless pipe sewer renovation, in which—in the case of a glass fiber-pipe liner system with UV or steam curing—it is known for installation purposes that a thick-walled PE film with a high density (HDPE or high density polyethylene) is applied to the inner wall of the pipe to be renovated (a sliding film), mostly in semicircular cross-section form. Subsequently, a flexible insertion tube (also known as tube liner or merely liner) is inserted into the pipe to be renovated (insertion process) and the insertion tube slides over the sliding film. On the one hand, this prevents the insertion tube from being damaged by the inner wall of the pipe or objects inside it and, on the other hand, this greatly reduces friction between the insertion tube and sliding film, thus facilitating the retraction of the insertion tube—a functioning similar to that of a shoehorn. In the glass fiber-tube liner system cured with UV or steam, such an insertion tube (tube liner) typically has an inner and outer tube with a carrier material (such as glass fibers) inserted between them that has been impregnated with reactive plastic resin. Some of the reactive plastic resins used are, for example, commercially available UP resins (polyester/unsaturated polyester resins), VE resins (vinyl ester resins) or EP resins (epoxy resins). For example, UP or VE resins are hardened with the help of photoinitiators, but heat can also harden them. The insertion tube is inflated inside the pipe until it presses tightly against the inner wall of the pipe or the sliding film in order to subsequently harden the resin—with ultraviolet light from a UV light source being pulled slowly through the pipe, for example. At the end, the inner film of the insertion tube is peeled off and removed. The layer with the carrier material is then exposed to the substances guided through the pipe.
Instead of the sliding film described above, a preliner (also known as preliner film) is often inserted into the pipe to be renovated—in particular in the case of the synthetic fiber-tube liner system with warm water or steam curing. A preliner, typically made of high density polyethylene (HDPE), is a thick-walled film that lines the pipe completely and also prevents direct contact of the tube liner with the inner wall of the pipe. To accomplish this, the preliner is directly laid on the inner wall of the pipe. Afterwards, the tube liner is inserted into the pipe (insertion process) or inverted (inversion process). The preliner prevents, for example, the tube liner's plastic resin from adhering to the pipe wall and the still unhardened resin from making contact with dirt and water. Furthermore, the preliner film also prevents the resin from seeping out of the sewer pipe renovation system and contaminating the soil and ground water. The preliner film also protects the feeds from penetrating excess resin so no resin plugs and obstructions can form. Additionally, during the insertion process, a preliner takes over a function that reduces the sliding friction, similar to the one described for the sliding films described above, for the tube liner that will be inserted. In this case, the low coefficient of friction between the sliding film or preliner and the outer film of the tube liner is what matters. As a result of this, the insertion tube or tube liner is not damaged by the pipe's inner wall or objects inside the pipe when it is inserted into the pipe and, on the other hand, the friction between tube liner and sliding film or the preliner is very low, thus facilitating retraction of the tube liner. Therefore, the preliner can also be called a sliding tube.
The disadvantage of the HDPE film used as a sliding film or preliner mentioned above is, among other things, its required thickness of approx. 500 μm to 800 μm or more, which translates into a large quantity to maintain more or less its mechanical stability and strength. Otherwise, the sliding film or preliner would develop cracks owing to the mechanical stresses inside the pipe caused especially by sharp or rough uneven surface irregularities and would no longer be able to fulfill its purpose.
In addition, the known HDPE film has a very large elongation capability that can even reach 200%. As a result of this, an “accordion effect” can occur when the tensile force is removed (i.e. the film contracts when this occurs). When, during the relative movement of the insertion tube with respect to the sliding film or preliner, the latter is stretched and finally the tensile force exerted by the insertion tube subsides, the sliding film or preliner can contract once again, in which case there is a risk that the sliding film or preliner and the insertion tube will create an obstruction in the sewer pipe to be renovated.
On the other hand, HDPE reaches maximum strength during sudden dynamic elongation. With further pulling (for example, with an even more forceful jerk), the film will start yielding irreversibly until it tears. Therefore, there is the danger of an uncontrollable retraction of the insertion tube, which therefore can get stuck in the pipe and may not be easily removed or properly inserted any longer.
Furthermore, the materials currently used in the PE films in the state of the art, especially HDPE films, are inadequate with respect to their mechanical properties such as (additional) tear strength, tensile strength, impact strength or puncture resistance. Finally, the PE films used as preliner film in the state of the art have no barrier effect either against resin components such as styrene, for example.